I think Cross-play is a good thing for gamers and developers.
Is also good for Auna, the ISP company I pay so I can play online games.
For gamers is good because more players mean (hopefully) short matchmaking times. It also open the possibility of maybe my cousin is playing Minecraft in the PS4, I buy the windows store version and play with him.
For developers is better (but can make things harder) because it means a obscure game can have a bigger public. So this multiplayer game can have a lot of players in PS4 despite selling poorly in the PS4, because theres a lot of PC gamers.
For Aunacable, my ISP, is good, because to play multiplayer games online, I pay then money, and they make sure that my TCP/IP packets get to the backbone of the internet, and the TCP/IP packets of the other players (in this peer2peer game architecture where the games are hosted in the consoles) meet my machine.
Is not a perfect thing.
For Nintendo, is a challenge, because they want a environment where is simply impossible for kids to get exposed to anything nasty.
For developers is a challenge because a more complex environment and problems that are hard to replicate, different schedules and requirements, trying to meet different opposite demands.
I don’t think theres much of a downside for the ones like Sony and Microsoft. They of course are the ones making complicated systems the developers would have to meet. But they already do that, probably,for other things, is basically their role.
Cross-play should be encouraged for small games, and games like Minecraft where you want the whole family to at least be able to play together. I don’t see it has neccesary in games like Battlefield, but seems to be a nice-to-have feature in games like “Car Footbal” and “Medieval Fighting Guys Game”.